It is known that strain is applied to a channel region under a gate electrode for the purpose of high speed operation of a MOS transistor. For example, the mobility of a carrier moving in the channel region is increased by disposing a film to apply a tensile stress in the side portion of the channel region in an NMOS transistor or by disposing a film to apply a compressive stress in the side portion of the channel region in a PMOS transistor.
It is known that recesses are disposed in source/drain regions to apply strain to the channel region and heteroepitaxial films of carbon doped Silicon (Si1-yCy):0<y<0.03 or silicon germanium (Si1-xGex) are disposed in the recesses. In this context, “Si1-yCy” or “carbon doped silicon” refers to single crystalline carbon doped silicon, in which mole fraction y is from 0 to 0.025. Also, Carbon contained silicon refers to polycrystalline or amorphous carbon doped silicon layer, because mole fraction y is not defined clearly due to polycrystalline or amorphous structure.
The lattice constant of Si1-yCy is smaller than that of silicon in a relaxed state. Therefore, regarding a Si1-yCy film epitaxially grown based on lattice matching with a (001) face of a silicon substrate, the lattice is strained in the direction of growth and a (110) face of the side surface of the channel region is contracted in a direction perpendicular to the substrate surface.
The Si1-yCy film is formed in such a way that the concentration distribution of carbon (C) becomes uniform in a film thickness direction. In order to make the concentration distribution of carbon (C) in a film thickness direction uniform, for example, monomethylsilane (MMS) in helium introduced into a reaction chamber is set at 20%.
In the case where the Si1-yCy films in the source/drain regions are also formed above the surface of the channel region, it is known that the Si1-yCy films are formed while a carbon gas is reduced gradually or stopped in the accumulation portions of Si1-yCy above the surface. Consequently, in the source/drain regions above the channel region, the C concentration is reduced gradually or becomes almost zero with decreasing proximity to the channel region.